We characterized the first RGB SiPMs with Ultra-High-Density cells (RGB-UHD) manufactured at Fondazione Bruno Kessler (FBK), Trento. These devices employ narrow trenches to separate the SiPM microcells and feature extremely high cell density, very low gain and correlated noise and ultra-fast recovery time. Such characteristics are of great interest in applications that require high dynamic range and/or good resistance to radiation damage, such as the CMS ECAL/HCAL upgrade.

The devices have cell pitch of 7.5 $\mu m$, 10 $\mu m$ and 12.5 $\mu m$, are arranged in a honeycomb configuration and have a circular active area with a 1.5 mm diameter. The cells have an hexagonal shape, thus, for example, the area of the 10 $\mu m$ cell is 87 $\mu m^2$. The cell density is 20500, 11500 and 7400 cells/mm$^2$, for the three cell sizes, respectively. For each cell, there are four layout splits, differing in one very critical parameter, which is the distance of the active area from the cell border and from the isolating trenches. We call these split D1…D4, ordered from the most challenging layout, D1, to the most conservative one, D4. Depending on these splits, the fill factor (FF) of the cells varies between 57% and 33% and between 68% and 47%, for the 7.5 and 10 $\mu m$ cells, respectively.

The automatic IV measurements showed that all cell sizes and layout splits were working in Geiger mode. We focused on the characterization of the cells with a 7.5 $\mu m$ and a 10 $\mu m$ pitch. The measured Gain was very small, in the order of $3.5\times 10^4$ and $4.5\times 10^4$ per 1 V of over-voltage ($V_{OV}$), for the 7.5 $\mu m$ and the 10 $\mu m$ cells, respectively. These values correspond to a cell capacitance ($C_T$) of 4.5 fF and 5.7 fF for the two cell sizes. The low value of $C_T$ allowed obtaining a very fast cell recovery time-constant, which was in the order of 3.5 ns and 4.5 ns, for the 7.5 $\mu m$ and 10 $\mu m$ cells. The primary, Poisson-distributed DCR measured at 20 $^\circ$C with the D3 splits was 80 kHz/mm$^2$ and 200 kHz/mm$^2$, for the 7.5 $\mu m$ and 10 $\mu m$ cells, respectively.

Thanks to the low gain, we measured extremely low correlated noise probability ($P_{CN}$), which represents the sum of afterpulsing and optical crosstalk probabilities. For both cell sizes, $P_{CN}$ was below 10% at 6 V over-voltage, resulting in ENF of $\sim$1.1. We also carried out a first PDE measurement at 515 nm on the 7.5 $\mu m$ and 10 $\mu m$ D1 cells, obtaining a PDE of 13.5% and 30% at 7 $V_{OV}$, which are remarkable values, considering the cell sizes. Finally, using a pulsed light source, we demonstrated single photon resolution at 20 $^\circ$C for both cell sizes.

In conclusion, we demonstrated the functionality (with a pulsed light source) and characterized the properties (in the dark) of the RGB-UHD SiPM technology, featuring an ultra-high density of cells, extremely low gain, very fast recovery time constant and good PDE. We expect that the UHD characteristics will reduce the effects of radiation damage on SiPMs. The fast recovery time and the high cell density reduce the PDE loss due to cells busy because of the increased DCR, while the low gain reduces afterpulsing and power consumption. In the conference presentation, we will describe the RGB-UHD technology and we will report in detail on the experimental characterization of the different cell sizes and layout splits.

Fondazione Bruno Kessler (Trento)

INR RAS (Moscow); University of Notre Dame (Notre Dame)